In recent years it has become obvious that the generation of greenhouse gases leads to global warming and that further increase in greenhouse gas production will accelerate global warming. Since CO2 (carbon dioxide) is identified as a main greenhouse gas, CCS (carbon capture and storage) is considered as one of the potential major means to reduce the release of greenhouse gases into the atmosphere and to control global warming. In this context CCS is defined as the process of CO2 capture, compression, transport and storage. Capture is defined as a process in which CO2 is removed either from the flue gases after combustion of a carbon based fuel or the removal of and processing of carbon before combustion. Regeneration of any absorbents, adsorbents or other means to remove CO2 from a flue gas or fuel gas flow is considered to be part of the capture process.
Backend CO2 capture or post combustion capture is a commercially promising technology for fossil fuelled power plants including CCPP (combined cycle power plants). In post-combustion capture the CO2 is removed from a flue gas. The remaining flue gas is released to the atmosphere and the CO2 is compressed for transportation, and storage. There are several technologies known to remove CO2 from a flue gas such as absorption, adsorption, membrane separation, and cryogenic separation. Power plants with post-combustion capture are the subject of this invention.
All known technologies for CO2 capture require relatively large amounts of energy. Due to the relatively low CO2 concentration of only about 4% in the flue gases of a conventional CCPP, the CO2 capture system (also called CO2 capture plant or CO2 capture equipment) for a conventional CCPP will be more costly and energy consuming per kg of captured CO2 than the ones for other types of fossil power plants, like coal fired plants, which have a relatively higher CO2 concentration.
The CO2 concentration in the CCPP flue gas depends on the fuel composition, gas turbine type and load and may vary substantially depending on the operating conditions of the gas turbine. This variation in the CO2 concentration can be detrimental to the performance, efficiency, and operatability of the CO2 capture system.
To increase the CO2 concentration in the flue gases of a CCPP two main concepts are known. One is the recirculation of flue gases as for example described by O. Bolland and S. Saether in “New Concepts for Natural Gas Fired Power Plants Which Simplify the Recovery of Carbon Dioxide” (Energy Conyers. Mgmt Vol. 33, No. 5-8, pp. 467-475, 1992). Another one is the sequential arrangement of plants, where the flue gas of a first CCPP is cooled down and used as inlet gas for a second CCPP to obtain a flue gas with increased CO2 in the flue gas of the second CCPP. Such an arrangement is for example described in the US20080060346. These methods reduce the total amount of flue gas and increase the CO2 concentration, and thereby reduce the required flow capacity of an absorber, the power consumption of the capture system, the capital expenditure for the capture system, and increase the CO2 capture system's efficiency.